Migrant workers (not interviewed by the Guardian) watch television in a labour camp in Qatar. All photographs by Pete Pattisson

Balfour Beatty and Interserve, two of the UK’s largest construction companies, have been accused of a raft of labour abuses by migrant workers employed on large-scale projects operated by companies that the firms co-own in Qatar.

Labourers on construction sites operated by BK Gulf, co-owned by Balfour Beatty, and Gulf Contracting Company (GCC), co-owned by Interserve, allege that they have been exploited and mistreated by labour supply companies hired by the firms to furnish construction sites in Doha with cheap manual workers.

The alleged abuses include erratic or reduced payment of wages, passport confiscation, workers entering employment with high levels of debt bondage, and pay levels below those agreed when workers were recruited in their home countries. The workers spoke of a culture of fear and intimidation, with threats of arrest or deportation if they stepped out of line.

The Guardian spoke to more than a dozen migrant workers, all of whom requested to remain anonymous due to fears of losing their jobs.

One labourer, employed by a supply company but working for GCC, which is 49% owned by Interserve, said he repeatedly asked his employer to let him leave Qatar after his salary was cut by 20%, but he was forced to stay. “I went to the [manager] and said, ‘I will give you the money for my plane ticket. Send me home.’ But he said, ‘Until and unless the contract finishes … you can’t go.’”

Labourers employed by labour supply companies but working for BK Gulf at Qatar’s national museum said they were earning a much lower salary than promised when they were recruited in Nepal. Some claimed their basic salary was only £135 a month. They said their wages were cut if they missed work due to illness, and they had to pay their own medical fees above £20.

“They withhold the first two months of salary … they do this to everyone,” said one worker from Nepal. “When I came here I was told by the agent that I could earn around 1,700 rials [£330] a month [with overtime]. If I had known I would only get 800 rials, and 200 rials for food, I definitely wouldn’t have come.”

Labourers working for BK Gulf at the museum praised safety management on the site, and two workers employed directly by GCC, building a office tower in Lusail City, said they were happy with the conditions of their labour camp. However, despite widely reported problems with the abuse and exploitation of migrant workers in Qatar and elsewhere in the Gulf, these new allegations suggest that neither British firm has taken the necessary precautions to ensure supply chains are free of labour abuses.

Ray Jureidini, a professor of migration, human rights and ethics at the Research Center for Islamic Legislation and Ethics in Qatar, said: “Companies that use these workers contract only with the labour supply companies, not the workers. The workers are not on their books and they don’t look into the company they have contracted with. They don’t look at how workers were recruited, their salary or living conditions … whether they are trafficking victims. I call it the corporate veil.”

However, some labourers working for BK Gulf and GCC continue to live in dire conditions in camps in the desert. In one apparently windowless room with beds for eight workers, clothes were strung on lines between the bunk beds and cooking pots were scattered on the floor. Men working for GCC claim there are up to 13 men sleeping to one room.

Qatari labour regulations state that there should be no more than four workers to a room in residential units and workers should be given at least four metres of personal space in communal areas.

“There are no cupboards or anywhere to keep our clothes or any goods … we have to keep everything on our bed,” explained one worker.

“They only turn on the water [in our camp] for an hour. Five minutes in the morning, and one hour at night,” said one, who lives in a nearby camp. “The air-conditioning is installed but they don’t switch it on … there’s not even a lock on our door.”

British firms have not hesitated to join Qatar’s building boom, which is worth an estimated $220bn (£155bn) as the emirate gears up to host the World Cup.

Most foreign construction companies must work in partnership with a local company that owns 51% of the business. These local subsidiaries then commonly source workers through labour supply companies, small firms that hire out labourers to work for other companies.

Labourers on their way to work at Qatar’s national museum. Some workers (not pictured) allege mistreatment at the hands of their labour supply company.

Balfour Beatty and Interserve said they were working within the parameters of Qatari law and rigorously monitored their labour supply companies to ensure good practice.

“BK Gulf WLL, in which Balfour Beatty has a 49% share, provides conditions for its workforce which go over and above local regulations and laws,” Balfour Beatty said.

“BK Gulf requires all of its labour supply companies and subcontractors to meet a selection criteria and code of conduct which includes requirements around operative working conditions. The company actively monitors its supply chain to ensure these standards and criteria are being met.”

It said it was reviewing labour supply companies and performing spot checks to ensure standards were being met.

Interserve said: “We are fully committed to supporting and protecting the health, safety and welfare of our employees and those working as part of our supply chain. We take these allegations extremely seriously and will investigate them thoroughly. If corrective measures are required, we will ensure they are implemented immediately.

“We only recruit through approved recruitment agencies, which we have assessed to ensure they meet our own high standards, industry best practice and comply fully with the Qatar labour laws.”

Both companies denied that they retained passports as a way of restricting workers’ movements. They said they offered to store them with the workers’ permission.

It is the market norm across the Middle East for workers to pay large fees to agents in their home countries, according to Balfour Beatty.

The company said the maximum number of workers to a room was seven.

Qatar: how have conditions for migrant workers changed in nine key areas?

“The importance of companies mapping out their sub-contracting recruitment chain is absolutely critical and contractors have to accept that if workers are on your site then they are your employees, even if they are hired by someone else,” said Wells.

“Construction is an industry dependent on working capital and most of these labour supply companies used by big companies are under-capitalised.”

“It is not enough for British companies to say they are operating in line with local norms when those norms have been criticised by human rights groups or to say they are working in compliance with local labour laws that have received widespread condemnation and fall short of the standards set by international bodies like the ILO,” said Damian Collins, a Conservative MP who campaigns for reform of football’s world governing body Fifa.

The Business and Human Rights Centre said companies operating in the Gulf are likely to face growing pressure to ensure they have oversight into worker conditions all the way down their sub-contracting chains.

On 1 April, a clause in the UK’s Modern Slavery Act came into effect that requires large British companies to begin producing public statements outlining the actions they are taking to ensure their supply chains are free of slavery.

“It will be interesting to see the modern slavery statements that British companies in the Gulf produce, and whether they will avail themselves of the convenient loophole that the UK government placed in the act, which could avoid them having to report on the actions of their subsidiaries abroad,” said Aidan McQuade, director of Anti-Slavery International.

Rajan’s story

The journey from the glistening towers of central Doha to the vast labour camps, which house tens of thousands of workers, takes less than an hour, but in that time you travel from the first world to the third.

As you turn into the camps, the four-lane highways give way to pot-holed streets, lined with heavy goods vehicles, workshops and abandoned cars covered in dust.

Rajan* emerges from the shadows and slips into our car. He is too afraid to let us into his camp, in case his boss finds out.

Rajan is employed by a company that supplies workers to Doha Festival City, a giant shopping mall under construction north of Doha.

According to Interserve, the contract for the shopping mall is worth £325m.

Last summer, the supply company that employs Rajan cut the basic salary of many of its workers from 1,100 rials a month to 900, to match the lower salary of an incoming batch of workers, Rajan says. Most of the workers who had their salaries slashed have returned to Nepal, but Rajan and a colleague remain in Qatar.

When his salary was cut, Rajan asked his boss to let him return to Nepal, but he refused. Rajan said he submitted two letters of resignation, even offering to pay for his flight, but they were rejected.

“I don’t feel good,” said Rajan. “I have told the [manager] I want to go … but the company said, ‘There are no staff now so you have to stay here for two months and then you can go.’”

The company told him to stay put “or there might be problems”.

“I have already got a warning,” explained Rajan. “Three months back we were not given our salary on time … we held a strike … the [Qatari] sponsor came and threatened us and said it was the last warning. We can’t do anything now.”

One labourer who had been working for the same supply company, but not on the GCC site, said he was imprisoned then deported after a salary dispute with his employer.

Like all the labourers interviewed by the Guardian, Rajan says his employer confiscated his passport, which is illegal under Qatari law. As part of its labour reforms, the authorities increased the fine for companies that confiscate passports to 25,000 rials a passport, but the practice remains widespread.

*Name has been changed to protect identity

• This article was updated on 13 April 2016 to include further comment from Interserve.